1. Field of the Invention
The present invention relates to an improved portable wireless telephone (i.e. a walkie Talkie) capable of providing both cellular and cordless telephone services.
2. Description of the Prior Art
Conventionally, cellular mode portable telephones have become popular, which can allow users to place or receive telephone calls at any point throughout a cellular network. On the other hand, cordless telephones have also become popular, which are typically used in the home. The cordless telephone consists of a base unit connected to a user's telephone landline and a wireless portable telephone unit associated with this base unit. Therefore, the cordless telephone enables the user to place a telephone call at any point throughout the home.
A cellular telephone is convenient when the user goes out of the office or the home, since it allows the user to place and receive telephone calls at any point throughout a large metropolitan area. However, the cost of a cellular telephone call is expensive since cellular telephone calls are made by way of expansive cellular base stations and cellular switching equipment.
Furthermore, when the user returns home, the user may want to use a cordless telephone rather than a cellular telephone. In such a case, the user needs to possess both a cellular telephone and a cordless telephone. Since these two telephone are fundamentally different, the ways of their uses are also different and, therefore, it was troublesome to use these different type telephones properly.
In order to resolve such a problem, it has been proposed a wireless portable telephone which is constituted to integrate a cellular telephone and a cordless telephone into one combined telephone and is capable of providing both cellular and cordless telephone services, as disclosed, for example, in Japanese Unexamined Patent Application No. HEI 3-1621 which is originated from the U.S. patent application Ser. No. 249, 041 filed on Sep. 23, 1988 and patented as the U.S. Pat. No. 4,989,230 on Jan. 29, 1991, or in the U.S. Pat. No. 5,127,042 patented on Jun. 30, 1992.
Hereinafter, with reference to FIGS. 4 and 5, a typical conventional portable wireless telephone is explained. A reference numeral 122 denotes a portable wireless telephone unit. A reference numeral 123 denotes a cellular base station in a cellular telephone network. And, a reference numeral 124 denotes a base unit of a cordless telephone.
In the case where the user wants to use this portable wireless telephone unit 122 as a cellular telephone, the user manipulates a mode selecting switch to activate a cellular mode. By the selection of the cellular mode, the portable wireless telephone unit 122 is situated in a condition for waiting a slave signal sent from the cellular base station 123. On the contrary, in such a waiting condition, it is possible for the user to place a telephone call through the cellular telephone network.
When the user places a telephone call, or when a telephone call is received, the portable wireless telephone unit 122 is engaged with the cellular base station 123 by way of a cellular radio channel of the cellular telephone network as shown in FIG. 4.
Furthermore, in the case where the user wants to use this portable wireless telephone unit 122 as a cordless telephone, the user manipulates the mode selecting switch to activate a cordless mode. By the selection of the cordless mode, the portable wireless telephone unit 122 is situated in a condition for waiting a slave signal sent from the base unit 124 of the cordless telephone. In such a waiting condition, it is also possible for the user to place a telephone call by manipulating an operational portion such as switches or buttons.
In waiting or calling conditions, a voltage detecting unit provided in the portable wireless telephone unit 122 keeps monitoring a remaining voltage of an installed battery. Therefore, when the voltage detecting unit detects that the battery voltage is decreased down to an inoperative level, a low-voltage detecting signal is outputted. In response to this low-voltage detecting signal, a display unit indicates that the battery voltage is so lowered that an immediate charging operation is required. Furthermore, in a calling condition, a telephone call is forcibly hung up.
However, in the above-described conventional constitution, a service area of a cellular telephone network is generally wider than that of a cordless telephone. An electric power required for transmitting signals or data of the cellular telephone is fairly larger than that of the cordless telephone. Therefore, electric power consumption of the battery become larger in the cellular telephone mode.
A low-voltage detecting level of the voltage detecting unit is set relatively higher for detecting an inoperative level in view of a large electric power consumption in the cellular telephone mode, so that the portable wireless telephone unit 122 can be surely supplied with electric power an amount of which satisfies a regulation required for a cellular telephone.
For this reason, this portable wireless telephone unit 122 is completely deactivated when the battery voltage is lowered down to the predetermined low-voltage detecting level so as to disable its transmitting function even if it can be still used as a cordless telephone. Therefore, there was a problem such that the portable wireless telephone unit cannot be used once the low-voltage detecting signal is generated even if the remaining battery voltage is still sufficient enough for activating a cordless telephone mode.